1. Field of the Invention
The present invention relates to a quantum computer and method using physical systems in a thin film.
2. Description of the Related Art
The state of the nuclear spin of each rare earth metal ion in crystal has a coherence time (the time of holding a quantum mechanical superposition state) specifically long as a solid. This property is a most important one required for physical systems that realize, in a solid state, a quantum information processing device, such as a quantum computer, which expresses information using superposition of states of each physical system and executes information processing. Further, the state of the nuclear spin of each rare earth metal ion in crystal can be controlled and read, using light of low noise that has excellent controllability. Because of these properties, the state of the nuclear spin of each rare earth metal ion in crystal is extremely suitable for realization of a quantum information processing device in a solid state.
There is a method in which when using nuclear spins in crystal as quantum bits, the states of the nuclear spins of individual ions are used as quantum bits, and differences between the ions in transition angular frequencies unique to solids, which are distributed in inhomogeneous broadening and do not change with time (in the case of gases, the inhomogeneous broadening is based on the motion of individual atoms and molecules, and hence their transition angular frequencies vary momentarily because of changes in the quantities of their motion due to their collision), are utilized to adjust the frequency of light applied to the ions so as to individually operate the quantum bits. This method employs an optical resonator, and the individual quantum bits contained therein are made to resonate with a common resonator mode, therefore can be processed substantially completely in a frequency domain. Namely, when accessing individual quantum bits or coupling quantum bits, the positional relationship between the quantum bits in a real space is not so important, with the result that a quantum computer, which does not require strict microfabrication of, for example, electrodes, can be realized (see, for example, Opt. Commun. 196, 119 (2001)).
Another method has been proposed, in which differences in transition angular frequency between ions contained in crystal are utilized for accessing individual quantum bits, as in the above-described method, whereby strict processing of crystal and electrodes is not needed (see, for example, Phys. Rev. A, 71, 062328 (2005)). In this method, a group of ions having transition angular frequencies that fall within a certain range set in units of quantum bits is used as a single quantum bit. This structure aims to obtain a read signal of a higher level than in the case of using a single ion as a quantum bit. In this structure, the dipole-dipole interaction of ions is utilized for coupling quantum bits. Namely, the ion-ion interaction (dipole-dipole interaction) of ions providing a certain quantum bit, and ions providing another quantum bit and located adjacent to the first-mentioned ions, is utilized.
However, it is considered that in this method, if the number of quantum bits is increased, the number of ions providing one quantum bit is reduced and accordingly the level of the read signal is reduced. It is also considered that if the number of quantum bits is increased, the average distance between physical systems expressing two quantum bits is increased to thereby weaken the interaction for coupling. If only ions of higher interaction levels are selected, the number of ions providing one quantum bit is further reduced.
There is yet another method in which quantum bits are discriminated using differences in transition angular frequency between physical systems that provide the quantum bits.
In this method, three types of physical systems adjacent to each other which are interactive are arranged cyclically (see, for example, Science 261, 1569 (1993)). Physical systems of different types have different transition angular frequencies, and physical systems of the same type have the same transition angular frequency. When a light beam that resonates with the physical systems of one of the three types is applied to the entire physical systems, the physical systems of the one type are operated in parallel. This parallel operation is repeated while the frequency of the applied light beam is changed to change the type of physical systems to be operated. During executing the parallel operation, the physical system located at an end is operated independently of the other physical systems, and the fact that the transition angular frequency of a certain physical system changes in accordance with the quantum state of the physical system adjacent thereto is utilized, thereby executing reading and writing data.
Further, in this method, an atomic group (monomeric substance) forming a macromolecular chain is regarded as a physical system, and only two states, i.e., the excited state and ground state of the monomeric substance, are used as a quantum bit. Thus, there is no concrete description concerning a physical system that has a sufficiently long coherence time, and concerning a method necessary to use a system (such as a state of a nuclear spin) as a quantum bit.
As a method of realizing a practical quantum computer using a large number of quantum bits, it is desirable to use a method that satisfies the following conditions:
(1) Quantum bits have a sufficiently long coherence time;
(2) The method is advantageous in that, for example, it is not necessary to process crystal and/or form electrodes by strict microfabrication;
(3) A physical system group formed of a large number of physical systems can be used as a single quantum bit to obtain a read signal of high level; and
(4) A sufficient scalability in the number of quantum bits is realized.
However, such a method as satisfies the above conditions is not known at present.